As the semiconductor light emitting element, a superluminescent diode (SLD) has characteristics that it has a wide light emission spectrum width relatively close to that of a light emitting diode and emits light with a narrow radiation angle and a high intensity like the light emission state of a semiconductor laser.
The SLD described in Patent Literature 1 includes a linear ridge waveguide formed vertical to a cleavage end surface seen in a plan view, and a curve guide active layer provided so as to bend following the ridge waveguide. On the cleavage end surface, an AR (anti-reflection) film is formed in some cases. In the SLD having such a structure, most of light generated in an active layer located immediately below the linear ridge waveguide travels to the curve guide active layer. The light toward the curve guide active layer is split into light leaked due to the curve, light that is guided to the end surface (end surface on the opposite side of the cleavage end surface) and reflected thereon, and light absorbed while being guided. With such a structure, because the light leaked due to the curve and light reflected on the end surface on the opposite side of the cleavage end surface cannot return to the linear active layer, the laser mode oscillation is suppressed (see, for example, lower right column of p. 2 to upper left column of P. 3, and FIGS. 1A and 1B).
In short, the SLD has not a structure where light is resonated by being reflected on mirrors provided on both end surfaces but a structure where light is amplified by being transmitted through a one-way waveguide (stimulated emission is performed) unlike the normal laser diode (LD). The difference between them is that the spectrum width of the wavelength of output light of the SLD is much larger than that of the LD.
Patent Literature 2 discloses a semiconductor light emitting element (e.g., SLD) used in a fiber optic gyroscope, an optical communication apparatus, an optical application measuring apparatus, and the like. The active layer (luminescent layer) of this semiconductor light emitting element is formed of InGaAs, and includes a plurality of quantum wells including a plurality of barrier layers and a plurality of well layers. It has been known that the quantum efficiency of light emission is improved by providing at least one strained well layer in the plurality of well layers. Specifically, the active layer of the semiconductor light emitting element disclosed in Patent Literature 1 includes a plurality of well layers (a first well layer and a second well layer) in which the composition ratio of the material differs. Accordingly, the structure of the active layer has a different band gap between the first well layer and the second well layer. As a result, with this element, it is possible to achieve wide light emission spectrum characteristics with the center wavelength of approximately 800 nm to approximately 850 nm (see, for example, paragraphs 0082, 0091 to 0098, and 0207, and FIG. 3A).